Starting in Europe in the late 1800s and continuing today, birth rates have been declining in societies as they become more affluent, industrialized, and technologically advanced. In fact, fertility output is dropping so precipitously in some countries -- such as Japan, Germany, and Italy -- that their total population is now in decline. This is a major cause of concern in these countries, whose leaders try figure out, for example, how a shrinking workforce will be able to support a growing elderly population. Some governments have even launched programs that award their citizens money (payoffs?) to have more children.

Many things change when societies modernize, but it's not clear which factors directly cause the declining birth rates. Is it because birth control methods are more effective and readily available to people? Because parents no longer need children as extra hands on the farm? Or has raising children just become too expensive in modern societies?

There have been no shortage of reasons proposed, but unfortunately, many of these explanations are unconvincing. For example, it's not about better birth control technologies; these were developed long after birth rates started dropping in Europe. Also, the idea that children can be financial commodities to their parents is largely a myth: the cost of raising a child vastly exceeds what the child could ever pitch in, even in agrarian societies of the past.

A more reasonable possibility is that parents limit their family size in order to allocate more resources to each child. Parents may have adopted this strategy because the relative cost of raising children in modern societies has skyrocketed. For example, education is now perceived to be a vital component of a prosperous life, so parents don't let their children drop out of school to help support the family; indeed, they're more likely to encourage their children to pursue optional forms of education such as college. Modern societies also have higher standards for what parents are expected to provide: a safe home and neighborhood to grow up in, good health care, healthy food, and plenty of parental attention. This theory of allocating more resources to fewer children, however, doesn't explain why so many modern couples decide not to have any children at all; it's not because they can't afford it - usually it's wealthier people who decide to stay childless. Perhaps people who enjoy a great deal of freedom in their lives are more reluctant to let go of it.

The newest explanation for declining birth rates, called the kin influence hypothesis, focuses on how people's social networks have expanded. In traditional (pre-modern) societies, social networks were relatively small and consisted almost entirely of genetic relatives (kin). But because family ties are generally weaker in modern societies, our social networks have come to include a large proportion of non-kin (e.g., friends, co-workers). As a result, family members now constitute a smaller part of people's social interactions than any time in our evolutionary history. According to the kin influence hypothesis, this change is the critical factor in decreasing birth rates because family members encourage each other to have children, whereas non-kin don't. Without this pro-reproductive encouragement coming from family, people are less likely to have children.

Think about it in evolutionary terms: if an organism's ultimate "goal" is to pass down his or her genes -- to maximize one's reproductive fitness -- then it should be in each person's interest for their kin to have children as well. When your siblings, or your cousins, or your own children have kids, part of your genetic heritage is being passed down and surviving. Just as we tend to help relatives more than non-relatives - because doing so increases our own reproductive fitness - we should also try to influence family members to have children.

A classic example is when a parent says to her married daughter, "I'd really love to see some grandchildren before I die." Usually, though, these messages are more subtle. For instance, parents and other family members may promise to help with any childcare that's needed. Or they'll reward higher status to family members who have children and accord lower status to those who don't. If you think about it, kin influence is a standard feature of the family environment. From an early age, most of us are socialized to get married and have children when we grow up - we are raised to believe that this is something we should do. Much of this socialization comes from parents and other family members.

In traditional societies, where family makes up a majority of a person's social world, youngsters are probably exposed to pro-reproductive messages on a regular basis. But when family is more distant and our interactions with them less frequent, this encouragement loses much of its power. As a result, there are weaker norms, attitudes, and values toward having children. The farther away people move from their families and the less contact they have with them, the fewer children they produce.

We should be clear about one thing though: it's not that friends discourage each other from having children; they typically don't. It's just that friends have no direct incentive to encourage it. In most cultures, reproductive decisions are considered private issues, not something for friends to be meddling into.

Of course, other changes in society contribute to lower birth rates as well. Religion is one of the few cultural institutions that still encourage people to reproduce ("Be fruitful and multiply"), but this influence tends to diminish as societies modernize. Instead, society has created other rewarding goals to pursue -- e.g., financial success, a meaningful career, leisure activities -- which are often hindered by having children. The allure of these pursuits has only been getting stronger, perhaps at the cost of birth rates.

Whatever conditions cause people to limit their family size, once this norm is established it spreads readily to other places. Traditional cultures, after merely being exposed to practices such as contraception and smaller family sizes in the mass media, begin to adopt these practices themselves. Today, as the mass media infiltrates new areas of the globe and more countries begin to modernize, this has significant implications for world population growth. Despite the familiar doomsday scenarios of overcrowding and unsustainable resources, current projections indicate that these low-fertility norms will greatly reduce population growth in the decades to come.

Women in less modern societies may not have as much choice about with whom or when to reproduce.

A few years ago an African female medical student told me that if you suggest to a man to wear a condom you, "might get beaten because he'll think you're accusing him of having HIV". An African female social worker told me that she always makes sure to have a female condom with her "in case she gets raped".

You bring up a very good point. Greater freedom, especially greater freedom for women, is an important feature of modernizing cultures. We can definitely think of cases in which young women, who at one time would have been forced into slavery with a husband who would have had complete control over her, can avoid this fate today because of a higher profile of women’s rights in many parts of the world. In general, it also makes sense that greater freedom for women can lead to less childbearing, and I think this freedom may have contributed to some of the declining fertility in the United States over the last 50 years.

But women's freedom doesn’t account for what’s happening in many other countries as much, where birth rates have started declining but women still have very little power. For example, fertility has been declining in Bangladesh and other developing nations now for at least a couple of decades, but gender roles are still as traditional as ever there.

It also seems like the freedom-fertility effect could work the other way around. Certainly, declining fertility leads to greater freedom for women.

In the United States, you could also make the case that greater freedom for men leads to lower birth rates. Not only do women have more freedom to have children out of wedlock (typically leading to fewer children relative to married women), but today men are not pressured into marrying women that they accidentally impregnate. Whereas 50 years ago a man would be strongly pressured to marry his girlfriend who got pregnant, today men don’t have to be as careful (or as responsible).

I come from India, and because Hindu scriptures link having male children to gaining spiritual salvation, most families here try very hard to have sons. This obsession with sons has not only skewed the country's sex ratio, it has also lead to the widespread practise of aborting female foetuses -- more so because parents want to limit family size; and a two-child family is seen as the ideal.
This trend is strongest is parts of the country, and in socio-economic groups, where female education and workforce participation is not exactly low. I think the link between female reproductive rights and female freedom is very tenuous. Many educated and employed Indian women do not have control over when and how many children they will have; many educated women are also forced to abort their female foetuses.

AIUI the desire for many progeny in part arises from the desire to have one's retirement be one of comfort, and the more offspring one has the greater resources one has available when one becomes old ... at least in places where conventional financial retirement is not an option.

I found a paper - and alas, I can't remember the citation - that noted fertility rates fell with the rise in the number of people eligible for Social Security. People don't need to have kids "for their old age" anymore. Nor do they need to have six children in order to see two or three grow up. Nor do kids contribute much to family income. Nor do women "have" to either get married, or never ever have sex. Etc.

...not reproducing themselves.
Though I imagine there'd be considerable disagreement on just which specific people would be best left without progeny...
Sadly, I have a sneaking suspicion that among the ones who self-select to end their family's genetic lines, higher IQs are over-represented.
There's at least one good doctoral dissertation awaiting on this...